No. Not for any good reason though... I just don't need it.

Firstly, I use LibreWolf rather than Firefox. It still has Sync but it's disabled by default and I've just never felt any interest in enabling it.

I use KeepassXC for passwords, so I don't need Firefox to sync those.

I don't use bookmarks. I don't have an amazing memory or whatever I've just always found it more convenient to just search for whatever I'm trying to find, whether that's searching my history or google or whatever. Like how do you organise bookmarks? Trying to categorise pages that I may wish to refer to in the future would drive me nuts. I do use Zotero which is similar to bookmarks but only for academic research.

Even transferring things between devices. That might be nice but seriously I just search for whatever it was I was looking at on whatever other device.

All that remains is addons and themes and honestly it's kinda nice to start with a clean slate on every new desktop and add in themes and addons. Only takes a minute.